In precision speed control systems, such as those used in centrifuges, both a wide range of speed control and high accuracy in speed control resolution are desirable. A centrifuge may, for example, have to operate through a speed range of many thousands of rpm and to yet be able to attain and remain controlled at a speed within of only a few rpm of the desired value. One approach to meeting these control requirements has involved the use of phase-locked control loop type electronic systems. In such systems a digital reference signal that is indicative of the desired speed is compared in phase with a digital tachometer signal produced by increasing the motor speed; to generate a phase difference signal. The latter signal is then used to change the motor speed as necessary to drive the tachometer signal into frequency and phase match with the reference signal.
While phase-locked control loop type systems are adequate for many applications, they suffer from two serious problems. The first problem is the actual and desired speed signals must have individual frequencies which are quite close to one another before the phase-locked loop circuit can achieve a stable locked condition. This is the result of deficiencies in the phase comparators used in phase locked loop electronic control systems, coupled with the time lag phenomenon in speed control associated with the inertia of the motor, which give rise to instability when the motor attempts to attain the desired speed from too far away. Accordingly, speed control systems that utilize control loop techniques require electronic circuit design which is able to first bring motor speed within a capture range of the phase-locked control loop and then surrender control thereto. This greatly complicates the electronic circuitry and the cost of its design and manufacture.
A second problem is the existence of a common mode component in the signal produced by the phase comparator used in a phase-locked control loop system, a common mode component. The results is, the output signal of the phase comparator will vary in relationship to a percentage difference, rather than the absolute difference between its input signal frequencies. This relationship causes the stability and resolution of the speed control system to change with the frequency of the reference signal.
While it is possible in principle to design analog electronic control circuits which are less subject to the described problems, of digital phase-locked control loop circuits, such analog circuits create other characteristic problems. Analog circuits are, for example, subject to thermal instability as the result of changes in the ambient temperature and are strongly affected by tolerances of their comprising electronic components. As a result, analog speed control systems often require the use of precision electronic components costing significantly more, costly thermal compensation systems and elaborate calibration procedures. These reaons may make precision analog speed control systems prohibitively expensive.